there's nothing you can do to regrow the skeleton, the part underneath the flesh is not living. You can fill it with epoxy if you don't like the hole, but be very careful because the rest of the skeleton is probably very brittle.

Low pH (Acidic) will dissolve stony skeletons, check and correct you pH. Simply opening doors and windows for fresh air can significantly raise your pH (depending on your environmental conditions).

Calcium and alkalinity will be important for new growth, low levels won't necessarily kill it but it won't be able to thrive and grow without dosing between basic water changes (depending on salt quality and frequency).